[64.06] High Resolution 3D Simulations of the Impacts of Asteroids into the Venusian Atmosphere

D. G. Korycansky, K. J. Zahnle, M.-M. Mac Low (AAS)

We compare high-resolution 2D and 3D numerical hydrocode
simulations of asteroids striking the atmosphere of Venus.
Our focus is on aerobraking and its effect on the size of
impact craters. We consider impacts both by spheres and by
the real asteroid 4769 Castalia, a severely nonspherical
body in a Venus-crossing orbit. We compute mass and momentum
fluxes as functions of altitude as global measures of the
asteroid's progress. We find that, on average, the 2D and 3D
simulations are in broad agreement over how quickly an
asteroid slows down, but that the scatter about the average
is much larger for the 2D models than for the 3D models. The
2D models appear to be strongly susceptible to the
``butterfly effect'', in which tiny changes in initial
conditions (e.g., 0.05% change in the impact velocity)
produce quite different chaotic evolutions. By contrast the
global properties of the 3D models appear more reproducible
despite seemingly large differences in initial conditions.
We argue that this difference between 2D and 3D models has
its root in the greater geometrical constraints present in
any 2D model, and in particular in the conservation of
enstrophy in 2D that forces energy to pool in large-scale
structures. It is the interaction of these artificial
large-scale structures that causes slightly different 2D
models to diverge so greatly. These constraints do not apply
in 3D and large scale structures are not observed to form. A
one-parameter modified pancake model reproduces the
crater-forming potential of the 3D Castalias quite well.

This work was supported by NASA's Exobiology and Planetary
Atmospheres Programs. Image rendering was done using the
resources of UCSC Vizualizaton Lab. M-MML is partially
supported by a CAREER fellowship from the US NSF. This work
was partially supported by the National Computational
Science Alliance, utilizing the NCSA SGI/CRAY Power
Challenge array at the University of Illinois,
Urbana-Champaign.